Feat/signature verify

[collinc97]

Overview

• adds documentation for sign.verify opcode in Aleo instructions.
• adds documentation for signature::verify and s.verify operations in Leo.

Example Aleo instructions code

sign.verify sign069ju4e8s66unu25celqycvsv3k9chdyz4n4sy62tx6wxj0u25vqp58hgu9hwyqc63qzxvjwesf2wz0krcvvw9kd9x0rsk4lwqn2acqhp9v0pdkhx6gvkanuuwratqmxa3du7l43c05253hhed9eg6ppzzfnjt06fpzp6msekdjxd36smjltndmxjndvv9x2uecsgngcwsc2qkns4afd r1 r2 into r3;

Example Leo code

    transition verify_field(s: signature, a: address, v: field) {
        let first: bool = signature::verify(s, a, v);
        let second: bool = s.verify(a, v);
        assert_eq(first, second);
    }

Linked Issues

Closes #268

[ghost]

LGTM.